374 lines
14 KiB
Markdown
Executable File
374 lines
14 KiB
Markdown
Executable File
---
|
|
author: Alvie Rahman
|
|
date: \today
|
|
title: MMME1029 // Manufacturing
|
|
tags: [ uni, nottingham, mechanical, engineering, mmme1029, manufacturing ]
|
|
---
|
|
|
|
# Cost Modelling
|
|
|
|
Key issues in selection:
|
|
|
|
- Component function, including materials and shape, form, and assembly
|
|
- Manufacturing process may greatly affect material properties, such as yield strength
|
|
- Similarly, the material will likely decide the manufacturing process
|
|
- Cost of a material and manufacture
|
|
|
|
The main requirement for a product to be viable is
|
|
|
|
$$cost < price < value$$
|
|
|
|
Cost modelling equation:
|
|
|
|
$$C = \frac{mC_m}{1-f} + \frac{C_t}{n} + \frac{1}{\dot n} \left[ \dot C_{oh} + \frac{C_c}{L\cdot t_{wo}} \right]$$
|
|
|
|
# Shaping Processes
|
|
|
|
## Casting
|
|
|
|
- Can be used for large size range
|
|
- Molten metal poured into solid mould to give shape
|
|
- Heat removed leads to shrinkage
|
|
- We need to be able to melt the metal and handlethe molten metal
|
|
- Mould degradation by the liquid metal needs to be considered
|
|
- Heat flowing from the molten metal into the mould causes a drop in temperature so solidification
|
|
starts from outside inwards
|
|
- Rate of solidification depends on rate of heat flow into mould
|
|
|
|
### Types of Mould
|
|
|
|
- Expendable mould (sand, plaster, ceramic)
|
|
|
|
- The mould is used once, being broken to release the casting
|
|
- Can have multiple use or single use pattern (investment and lost foam casting
|
|
|
|
- Multiple mold casting
|
|
|
|
- Die casting (pressure die casting)
|
|
- Permanent mould casting (gravity die casting)
|
|
|
|
#### Sand Casting
|
|
|
|
|
|
|
|
- Wide range of metals can be cast
|
|
- Almost no limit to size and shape of casting
|
|
- Poorer tolerances than other proces, rough surface texture
|
|
- Slow
|
|
- Economic for a low number of castings
|
|
- Applications include cylinder blocks and large pipe fittings
|
|
|
|
#### Investment Casting
|
|
|
|
- A high cost process
|
|
- Used mostly for complex shapes, such as sculptures, jewellery, and gas turbine blades
|
|
- Can be used for a wide range of metals
|
|
- Very high precision and surface finish
|
|
|
|
1. Make a master die
|
|
2. Make wax pattern by casting wax into master die
|
|
3. Coat wax pattern with investment material
|
|
|
|
1. First with a slurry of water and fine ceramic to capture fine details
|
|
2. Then coat with stucco, which is a thicker coating for strength
|
|
|
|
4. Heat mould to melt wax out, bake and preheat mould
|
|
5. Pour in molten metal
|
|
6. Wait for solidification, break mould when done
|
|
|
|
#### Permanent Mould Casting (Gravity Die Casting)
|
|
|
|
- Mould cavity is machined into mating metal blocks
|
|
- Molten material poured into mould
|
|
- Mould material is cast iron, steel, bronze, graphite
|
|
- Mould must disassebmble without locking
|
|
- Mould is expensive but can be reused (typically around 25k times)
|
|
- Mould life is reduce by casting high meling point metals
|
|
- Good surface finish and dimensional accuracy
|
|
- Cooling is rapid therefore high production rates
|
|
- Example use is a piston
|
|
|
|
#### Die Casting (High Pressure Die Casting)
|
|
|
|
- Dies must be able to withstand high pressure
|
|
- 0.1 mm slits at parting lines provide escape for air
|
|
- Dies are made of expensive tool steels
|
|
- High volume production is necessary to justify costs
|
|
- Generally limited to low viscosity, low melting point, non ferrous metals like Al, Zn, Mg, and Pb
|
|
- Good surface finish
|
|
- Precision castings with thickness between 0.75 mm and 12 mm
|
|
|
|
### Design of Castings
|
|
|
|
- Distribute castings evently around parting planes
|
|
- Need to be able to get patterns out of moulds and casting out of moulds where applicable
|
|
- No re-entrants (complex multi-part moulds may be able to avoid this restriction)
|
|
- Draft angle between surfaces
|
|
|
|
- Need to be able to get solid patternout of mould in sand casting
|
|
- Need to be able to get solid casting out of mould in die casting
|
|
|
|
- Allow for shrinkage --- dimensions of casting mould/pattern needs to be made so that part is
|
|
desired size after shrinkage
|
|
- Avoid rapid change in section or direction:
|
|
|
|
|
|
|
|
### Solifidification of Metals
|
|
|
|
- How well the liquid fills detail depends on viscosity of liquid
|
|
- During freezing, latent heat of fusion is removed
|
|
- During freezing, material is a solid/liquid mixture
|
|
- There is a significant (~7 %v) shrinkage during solidification
|
|
- Heat flows down steepest thermal gradient so usually there is an actively cooled section
|
|
- Thin sections freeze faster than thick sections
|
|
|
|
### Castability
|
|
|
|
- Low melting point
|
|
- Low viscosity and surface tension
|
|
- Low solidification contraction
|
|
- Low thermal capacity and high conductibity
|
|
- Low solubility
|
|
- Not contaminated by air
|
|
|
|
## Deformation
|
|
|
|
When a metal is plastically deformed, dislocations move and multiply.
|
|
|
|
Annealed aluminium may have a dislocatio density of around 200 m per mm$^3$.
|
|
This is a very low amount.
|
|
A heavily cold worked piece may have a density of up to 270 km per mm$^3$.
|
|
|
|
As dislocation density increases, the dislocations impede the motion of other dislocations.
|
|
This means that to continue plastically deforming, more stress has to be applied.
|
|
|
|
The stress goes down towards the end of the graph due to the material necking, meaning the
|
|
material gets thinner.
|
|
This means that the engineering stress is lower as the true area is lower.
|
|
The true stress, however, is going up:
|
|
|
|
|
|
|
|
### Effect of Prior Deformation (*Work Hardening*)
|
|
|
|
|
|
|
|
See
|
|
[here](materials.html#work-hardening-and-cold-working)
|
|
for more information
|
|
|
|
### Effect of Temperature (Diffusion)
|
|
|
|
In an alloy, atoms tend to migrate from regions of high concentration to low concentration.
|
|
This is diffusion.
|
|
|
|
More information on diffusion [here](materials.html#diffusion).
|
|
|
|
### Annealing
|
|
|
|
Annealing is a process by which a component is heated to reduce work hardening.
|
|
|
|
|
|
|
|
These are diffusional processes and only occur at higher temperatures.
|
|
|
|
When the temperature of a material, $T > 0.55T_m$, it is said to be hot.
|
|
A material being worked on hot has its deformations eliminated as fast as they are created.
|
|
|
|
A material is said to be cold when $T < 0.35T_m$.
|
|
|
|
# Powder Processes
|
|
|
|
Poweders can plowflow if forces between them are low
|
|
|
|
With small amounts of binder, they can form "*plastic*" materials like clay.
|
|
|
|
A *slurry* can be formed with a liquid carrier (where there is enough liquid to separate particles).
|
|
In a slurry, often you want to reduce liquid content but avoid the particles touching or attracting
|
|
each other.
|
|
Adding a *deflocculant*[^d_deflocculant] results in the formation of a stable *slip*.
|
|
|
|
Making the powders is often quite expensive when you have a controlled size distribution.
|
|
|
|
## Clay and Ceramics
|
|
|
|
Clay is an abundant raw material but it needs to be milled and screen for a controlled size
|
|
distribution.
|
|
When mixed with water it forms a *plastic* material.
|
|
|
|
Structural clay products include bricks, tiles, and pipes.
|
|
Other proucts include whitewares such as porcelain, pottery, and tableware.
|
|
|
|
Ways to form the clay include pressing, isostatic pressing, extrusion, and machining.
|
|
|
|
Engineering ceramics (e.g. silicon carbide, alumina) are shaped with small amounts of binder ---
|
|
commonly pressed or isostatically pressed.
|
|
|
|
## Slip Casting
|
|
|
|
1. Pour slip into a mould (e.g. plaster of Paris)
|
|
2. The mould is extremely water absorbing. This results in the remaining part developing some
|
|
structural integrity.
|
|
3. Remove the mould and place in the oven to reduce water content.
|
|
4. Fire to harden
|
|
5. Add glaze and fire again.
|
|
|
|
Drying leads to shrinkage and potential cracking.
|
|
It also gives strength and allows for handling and maybe machining.
|
|
|
|
|
|
|
|
## Sintering of Metals and Ceramics
|
|
|
|
|
|
|
|
Atoms diffuse to points of contact, creating bridges and reducing the pore size.
|
|
Diffusion is driven by a desire to reduce the surface area as surfaces are regions of high energy.
|
|
|
|
## Powdering Metallurgy
|
|
|
|
- Competitive with processes like casting, forging, machining
|
|
- Used when the melting point is too high, a chemical reaction occurs at melting point, the part is
|
|
too hard to machine, or a very large quantity (on the order of 100 000) of the part is needed
|
|
- Nearly 70% of parts produced is by powder metallurgy
|
|
- Good dimensional accuracy
|
|
- Controlloable porosity
|
|
- Size range from balls in ball point pens to parts weighing 50 kg
|
|
|
|
Basic steps of powder metallurgy:
|
|
|
|
1. Powder production (commonly atomization) --- this is often a costly process and you must minimize
|
|
oxidation of the metal
|
|
2. Blending/mixing --- add binders to keep the particles together and lubricants to reduce damage to
|
|
dies and aid consolidation
|
|
3. Powder consolidation
|
|
|
|
- Shaping in a die
|
|
- 100-900 MPa of pressure applied
|
|
- Fast process as no heat needs to be removed
|
|
|
|
4. Sintering at $0.7T_m$ to $0.9T_m$
|
|
|
|
Shaping equipment has no requirement to be able to withstand high temperatures and the sintering
|
|
equipment does not have the need for complex designs.
|
|
This separates problems, making them easier to design.
|
|
|
|
The pressing equipment is costly but the time spent pressing is quite small, allowing for greater
|
|
throughput.
|
|
Additionally, the furnace can operate continuously and is simple and cost effective.
|
|
|
|
|
|
|
|
### Green Density
|
|
|
|
The *green density* is a fraction of the true density.
|
|
A low green density will result in high shrinkage on sintering.
|
|
|
|
## Moulding
|
|
|
|
Moulding is a shaping process used for viscous materials (typically polymers and glasses).
|
|
Here the material can hold a shape unsupported but not for very long or under even small stresses.
|
|
|
|
In order to mould a material we must raise the temperature above the glass transition temperature,
|
|
$T_g$.
|
|
At this temperature, the C-C bond in the chapolymer chain are able to easily rotate around each
|
|
other.
|
|
|
|
Large side chains or molecules on the main chain make it harder to rotate these bonds, making
|
|
$T_g$ higher.
|
|
Polar groups (e.g. chloride, cynaide, and hydroxide) have also hinder bond rotation.
|
|
|
|
More information about polymers
|
|
[here](https://notes.alv.cx/notes/uni/mmme/1029_materials_and_manufacturing/materials.html#polymers-1).
|
|
|
|
## Extrusion
|
|
|
|
Extrusion produced parts of constant cross section, like pipes and rods.
|
|
The process is used primarily with thermoplastics and 60% of polymers are prepared by extrusion.
|
|
|
|
|
|
|
|
## Blow Moulding
|
|
|
|
Blow moulding is a rapid process with low labour costs.
|
|
It produces hollow components that do not require a constant thickness, such as
|
|
bottles, petrol tanks, and drums.
|
|
Common materials to blow mould are HDPE, LDPE, PP, PET, and PVC.
|
|
|
|
There are three common types of blow moulding:
|
|
|
|
- Extrusion blow moulding
|
|
- Injetion blow moulding
|
|
- Stretch-blow processes
|
|
|
|
However, they involve the following stages:
|
|
|
|
1. A tubular preform, called a *parison* (a word I haven't been able to remember since GCSE) is
|
|
produced by either extrusion of injection moulding
|
|
2. The *parison* is transferred into a cooled split-mould
|
|
3. The *parison* is sealed and inflated to take up the shape of the mould
|
|
4. The moulding is let to cool and solifidies under pressure
|
|
5. The mould is opened and moulding is ejected
|
|
|
|
|
|
|
|
## Injection Moulding
|
|
|
|
1. Powder or pellets of polymer heated to liquid state (low viscosity)
|
|
2. Under pressure, the liquid polymer is forced into a mould through a *sprue*, a small opening
|
|
3. The pressurized material is held in the mould until it solidifies
|
|
4. The mould is opened and the part is removed by ejector pins
|
|
Selection was cancelled by keystroke or right-click.
|
|
|
|
|
|
|
|
Theromoplastics are most common in injection moulding.
|
|
A very high level of detail is attainable through this process and it produced little waste.
|
|
|
|
Similar to [Die Casting](#die-casting-high-pressure-die-casting), you must consider corners (avoid
|
|
sharp ones), draft angles (so you can get the part out), and section thickness (using ribs instead
|
|
is preferable).
|
|
|
|
Due to the high capital cost, injection moulding is only economical at high production volumes.
|
|
|
|
### Co-Injection Moulding
|
|
|
|
There is *sequential moulding* (one after the other) and *co-injection moulding* (together).
|
|
|
|
These processes reduce assembly costs by integrating the parts and can use low grade recycled
|
|
material for the inside of a component.
|
|
It also allows for a part have to have multiple colours.
|
|
|
|
This process requires special attention to be payed to shrinking/cooling.
|
|
|
|
## Rotational Moulding
|
|
|
|
Rotational moulding involves coating the insides of a heated mould with a thermoplastic.
|
|
It is a low pressure alternative to blow moulding for making hollow components and is used
|
|
for large components such as storage tanks, boat hulls, kayaks, and cones.
|
|
|
|
## Moulding for Thermosetting Polymers
|
|
|
|
There are two types:
|
|
|
|
a. Compression moulding
|
|
b. Transfer moulding
|
|
|
|
|
|
|
|
### Compression Moulding
|
|
|
|
For thermoplastics, the mould is cooled before removoal so the part will not lose its shape.
|
|
Thermosets, however, may be ejected while they are hot so long as curing is complete.
|
|
|
|
The process is slow but the material only moves a short distance and has lower mould pressures.
|
|
It also does minimal damage to reinforcing fibres in composites and it is possible to make large
|
|
parts.
|
|
|
|
More manual labour is required and has longer cycle times than injection moulding.
|
|
|
|
# Glossary
|
|
|
|
[^d_deflocculant]: a substance which, when added to scattered particles in suspension, causes a reduction in apparent viscosity. Deflocculants are substances which prevent flocculation by increasing zeta potential and therefore the repulsive forces between particles. (<https://digitalfire.com/article/deflocculants%3A+a+detailed+overview>)
|